Nickel-chromium steel having increased resistance to corrosion

ABSTRACT

A nitric acid resistant nickel-chromium steel which, when welded, effectively prevents the formation of intermetallic deposits at the grain boundaries in the weld area. The composition of this steel consists essentially of up to 0.15% carbon, 2-6% silicon, 0-6% manganese, 16-25% chromium, 10-25% nickel, 0.08-2% nitrogen, and the remainder being iron and unavoidable impurities.

ilnite @inies Pnieni Inventor Anton Baurnel lliapienberg, Austria Appl.No. 736,988 Filed June 14, 1960 Patented Oct. 26, 1971 Assignee Oebr.lBohler & (10., Alttiengeseilschait Vienna, Austria Priority June 19,1967 Austria A5670/67 NlClKElL-CllllltOMlUM STlElElL i-lAVlNGiNClltlEASlED RESiSTANClE TO CORROSHON 3 Oiniins, No Drawings [56]References Cited UNlTED STATES PATENTS 2,984,563 5/1961 Tanczyn2,820,708 1/1958 Waxweiler.

2,747,989 5/1956 Kirkby 3,337,331 8/1967 Ljunberg...

Primary Eraminer- Hyland Bizot Attorney1-iolrnan & Stern 75/128C 75/128C75/128C 75/128C remainder being iron and unavoidable impurities.

ABSTRAQ'E: A nitric acid resistant nickel-chromium steelNlClliEL-CHMOMRUM STEEL HAVING llNCRlEASlElD RESISTANCE T CORROSHON Thisinvention relates to a nickel-chromium steel which has in its unweldedand welded states an increased resistance to corrosion by reducingagents or agents which contain chlorine ions and particularly byoxidizing agents, such as nitric acid.

it is known that an austenitic nickel-chromium steel which containschlorine 15-25% chromium, 7-20% nickel, 0.005-0'.2% carbon, 0-4%molybdenum, 0.l-% manganese and 2.5-6% silicon has an increasedresistance to stress crack corrosion and that steels of substantiallythe same type exhibit an increased resistance to solutions of nitricacid. The practical use of these steels is limited, however, by the factthat they have a considerable disadvantage in the as-welded state. Inthe area which adjoins the seam weld, the welding heat induces theformation of a zone which is highly susceptible to intercrystallinecorrosion because the silicon-containing steel is removed at anincreased rate even in the absence of additional oxidizing agents, suchas hexavalent chromium or trivalent iron. in these areas, a while silicalayer is formed and the corrosion proceeds under this layer. Thissusceptible zone must not be confused with the known zone which issusceptible to intercrystalline corrosion in welded, nonstabilized, 8:18nickel-chromium steels having an increased carbon content. The formationof this latter zone is induced by a deposition of chromium carbide atthe grain boundaries. In the silicon-containing steels which are underconsideration in the present case, the carbon content is so low thatthis deposition does not occur. Metallographic investigations haveshown, however, that the welding heat induces in these high-siliconsteels, at a temperature of about 850, the formation of an intermetallicdeposit having highly increased chromium and silicon contents at thegrain boundaries, so that these elements are depleted from the regionsadjacent these grain boundaries. As these two elements are essential forthe materials resistance to attack by nitric acid corrosion, thedepleted regions ad joining the grain boundaries are more susceptible tonitric acid and a zone which is susceptible to intergranular corrosionis formed at a certain distance from the seam weld on both sidesthereof.

As welding of the steel is generally inevitable in the manufacture ofchemical equipment, the austenitic nickel-chromium steels which containsilicon and have an increased resistance to corrosion cannot be used inpractice unless the above problem involving in the welding of suchsteels is solved.

My investigations have shown that the formation of intermetallicdeposits at the grain boundaries in steels of this type at temperaturesof about 850 C. can be retarded and the increased corrosion adjacent tothe seam weld can be reduced, or avoided, if the steels contain nitrogenin an amount of 0.04-0.3711, preferably 0.08-0.2%.

The nickel-chromium steel according to the invention which has anincreased resistance to corrosion in its unwelded and welded states isthus broadly characterized in that it contains up to 0.25% carbon, -10%,preferably 2-8%, silicon, 0-10% manganese, 13-30% chromium, 4-3070nickel, 0-10% molybdenum and 0.04-0.3% nitrogen, balance iron and inevitable impurities.

According to the preferred embodiment of the invention, a steel which inits unwelded and welded states has a particularly high resistance toreducing agents or agents which contain chlorine ions contains up to0.15% carbon, 2-6 percent silicon, 0-6 percent manganese, 16-25%chromium, 10-25% nickel, 2-6% molybdenum and 0.080.2% nitrogen, balanceiron and inevitable impurities.

Without nitrogen, a steel of this type would also have a strong tendencyto deposit intermetallic phases adjacent to the seam welds so that itssusceptibility to an attack by agents such as sulfuric acids would beincreased.

Best results as to the resistance to oxidizing agents, particularlynitric acid, in the unwelded and welded states of the steel, will beexhibited by a steel which contains up to 0.15% carbon, 2-6% silicon,0-6% manganese, 16-25% chromium, 10-25% nickel and 0.08-0.2% nitrogen,balance iron and inevitable impurities.

A number of steel specimens having different nitrogen contents weresubjected to welding tests and to comparative corrosion tests in boilingconcentrated nitric acid, and dilute hydrochloric and sulfuric acids.These tests revealed a clear superiority of the nickel-chromium steelsaccording to the invention. The same result was obtained when steelspecimens were quenched from 1,l00 C. in water and subsequently temperedfor 5 minutes to 10 hours. These specimens were tested for their notchimpact strength relative to the tempering time and for their weight losswhen submerged in boiling concentrated nitric acid for three times eachtime for a period of 48 hours. Best results were obtained with steelswhich contain 0.1 1% or 0.16% nitrogen. 1f the nitrogen content exceeds0.22%, it appears necessary to avoid a formation of chromium nitrides,which particularly adversely affects the mechanical behavior.

Composition of Steels:

The filler material used when welding is the same as the composition ofthe nickel-chromium steel being welded, and moreover, the iron in thetiller material contains up to 20% delta ferrite as based on the weightof the filler material.

What is claimed is:

l. A nitric acid resistant nickel-chromium steel consisting essentiallyof up to 0.15% carbon, 2-6% silicon, up to 6% manganese, 16-25%chromium, 10-25% nickel, ODS-0.2% nitrogen, the remainder being iron andunavoidable impurities wherein, during welding, the formation ofintermetallic deposits at the grain boundaries is effectively minimized.

2. A nickel-chromium steel as set forth in claim 8, which contains up to20% delta-ferrite and constitutes a filler metal for welding.

3. A nickel-chromium steel as set forth in claim 1 wherein said fillermetal is in the welded state.

2. A nickel-chromium steel as set forth in claim 8, which contains up to20% delta-ferrite and constitutes a filler metal for welding.
 3. Anickel-chromium steel as set forth in claim 1 wherein said filler metalis in the welded state.